华南海岸沙丘岩的特征及其形成发育模式

THE CHARACTERISTICS AND THE DEVELOPMENT MODEL OF DUNE ROCKS ON SOUTH CHINA COASTS

海岸沙丘岩是一种热带、亚热带干旱及季节性湿润地区特有的碳酸盐胶结的风成碎屑岩。本文对华南沿海沙丘岩的沉积结构、沉积构造特征,地球化学与古生物特征,及其胶结类型、成岩机制和发育模式进行了较系统研究,并讨论了与海滩岩的区别。

Dune rocks are aeolian sands cemented by calcium carbonate under subaerial conditions. They have been found on many of the coastal belts of Fujian, Guangdong and Hainan Provinces in South China. The grain composition of the dune rocks is mainly quartz sands and shell fragments; The quartz sands are medium and fine sized, relatively well sorted and positively skewed; Their surface texture formed in aeolian environments is characterized by dishshaped depressions, meniscus depressions and V-shaped depressions with rounded edges; The most common bedding type of the rocks is large (thickness>1.5m) steeply dipping (32—40°0) cross strata (planer and convex upward); Mg and Sr content are very low in the rock chemical composition which is classified into low Mg and low Sr category; The typical species of microfossils in the dune rocks are mainly freshwater ones and lack of typical saltwater or semi-saltwater ones with incomplete assemblage of marine species; The cement minerals in the rocks are mainly low-Mg calcite and the common cement fabrics are meniscus cement and gravitational cement in response to impermanent water in vadose zones. Therefore, the dune rocks may be apparently distinguished from the beach rocks. The cement processes in South China coastal dune rocks always take place on near coastal belts and upper layers of the rocks themselves resulted from the dissolution of shell fragments, the infiltration of salt spray and the strong evaporation on the coastal dune surface. The process of development and formation of the dune rocks may be divided into 3 stages: (1) the material accumulation, (2) aeolian sands deposition, and (3) diagenic cementation. These three stages took place alternatively at the same time.